The Mohs Scale Explained: Stone Hardness, Examples and Pitfalls

A ranking by scratching

In 1812, the German mineralogist Friedrich Mohs proposed a simple scale for comparing minerals: you test which one scratches which. He chose ten reference minerals, from the softest, talc (1), to the hardest, diamond (10). A mineral with a hardness of 5 scratches those of 4 and below, and is scratched by those of 6 and above. This is why the scale is still used by geologists, gemmologists and enthusiasts.

One essential point: the scale is ordinal. It gives a rank, not a measurement. Diamond (10) is not “a little harder” than corundum (9): it is far harder, whereas the gaps between the lower degrees are much smaller. Other scales (Knoop, Vickers) measure hardness in absolute values.

The ten degrees and their examples

What the scale says... and what it does not

• Hardness does not mean strength. Diamond resists scratches, but it has cleavage planes: a well-placed blow can split it. Topaz (8) has perfect cleavage; so does kunzite (6.5 to 7).
• Toughness (resistance to impact) is a separate property. Nephrite, one of the two jades, has modest hardness but is extremely tough.
• Rocks and mixtures: lapis lazuli, jasper, larvikite and nuummite are rocks, which combine several minerals. Their hardness varies from place to place; the values given are averages.
• Glass and treatments: obsidian is a natural glass, goldstone a manufactured glass; a stone with a treated surface (aqua aura) can wear away long before the quartz it sits on.
• Sources differ: for the same mineral, tables sometimes give a 0.5 point difference (for example 6 to 6.5 or 6.5 to 7).

The rule of 7 for jewellery

Air, sand and dust contain quartz, which has a hardness of 7. Any material softer than 7 will therefore end up scratched by contact with dust, a key or a worktop. For jewellery, this gives a useful guide:

• Rings and bracelets, exposed to friction: prefer 7 or more (quartz, tourmaline, garnet, beryl, sapphire).
• Pendants, earrings, necklaces: a softer stone may be suitable, as it is less exposed to knocks.
• Soft stones (selenite, malachite, fluorite, amber, lapis, turquoise): reserve them for jewellery worn with care or for decorative objects.

See the pages on stone rings and stone bracelets for concrete choices. Keep soft stones apart from hard ones, otherwise the harder ones will scratch the softer ones.

A scratch test: with care

You can estimate a stone’s hardness by trying to scratch it with objects of known hardness: a fingernail (2 to 2.5), a copper coin (3), a steel blade (5.5). The test leaves a mark: do it on a discreet area, never on a precious stone or a piece of jewellery, and use the tip of a softer object first. Many stones cannot be identified by this test alone. See the tutorial testing a stone at home for a gentle method.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Care and safety for stones.